Diffusion

Simple freshwater animals that do not have excretory organs osmoregulate by ionic regulation. These include sponges, coelenterates, and bivalve molluscs. They are also among the most dilute of animals. By maintaining a low gradient between themselves and the environment, they reduce their energy requirements. Freshwater Hydra and medusa, such as Craspedacusta, transport ions across their cell membranes and deposit them into the gastrovascular cavity. By moving sodium, water diffuses from the tissues into the gut. The excess water, which is hypertonic, is then eliminated by opening the mouth to the gastrovascular cavity.

Simple marine organisms also actively regulate their ionic concentrations. Some of these animals do not regulate the osmotic concentration of their body fluids. They simply shrink and swell with changes in the environment. These animals are called osmoconformers and their internal fluids are isosmotic with the environment because the water content changes with the environment.

Most marine animals are osmoregulators. That is, their internal fluids usually have a different osmotic concentration than the environment. These animals either move large amounts of water as the needs dictate. A few animals can osmoregulate, but do not move large quantities of water. They shift the osmotic, but not the ionic, concentration of their body fluids as the environment changes. An example of animal that does this well are sea urchins that use amino acids to osmoregulate. As salinity increases, so does the concentration of amino acids in their body fluids.

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